1. Field of the Invention
This invention relates to improved abrasive grains, to the manufacture of abrasive grains and to articles composed of abrasive grains.
2. Description of the Prior Art
Two general types of man-made aluminous abrasives are well known in the art, being identifiable respectively as fused and sintered. Fused aluminous abrasive grains are produced commercially by crushing ingots of fused aluminous compositions and separating the resulting pieces, or grains, into the various sizes required by means of a series of sieves. Sintered abrasive grains are produced by milling an aluminous composition to a fine powder, wetting to produce a plastic msas, extruding the plastic mass into rods of desired cross-sectional size, cutting or granulating the rods and firing the grains to elevated temperatures below the melting point, or in place of extruding, pressing into a block which is then crushed to produce granular particles prior to firing.
The friability of the individual fused or sintered abrasive grains, that is, the ease with which they break down due to impact, can be varied by controlling the composition and the final size of the crystals making up the abrasive.
In the case of a fused abrasive, the size of the crystals is generally a function of the cooling rate from the molten state -- slow cooling promoting larger crystals. In the case of a sintered abrasive, the size of the crystals is generally a function of the amount of grinding performed on the raw materials.
This difference plus the added strains and shear planes developed during crushing makes a fused abrasive generally more friable than a sintered abrasive of equivalent composition.
The lesser friability of sintered abrasives results in poorer grinding characteristics on carbon steel and cast iron than found when fused abrasive is used. The problem manifests itself in the form of loading of the grinding wheel which means a coating is formed on the grinding surface by metallic swarf, or bits of metal. As the coating builds up, the grinding action slows. To counter this effect, extremely high pressures have been applied in the past to sintered abrasive wheels to break down the coated layer and expose a fresh surface.